If you do not receive any other stimulus payments by January 15, 2021, then the stimulus payment for which you are eligible to receive will be entered on your 2020 federal tax return as a tax credit on Form 1040 Line 30.
The TurboTax program will ask about stimulus payments in the Federal Review section of the program.
Don't worry about the past or 2019.
The IRS will stop mailing Stimulus checks on 1/15 so if you don't get it by the time the efiling opens at the end of the month you can apply for the missing money on the 2020 return.
The Stimulus checks are really based on your 2020 tax return. Any amount you already got will be reconciled on your 2020 return line 30. If you didn't get a Stimulus Check (and qualify) or you qualify for more it will be added to your 2020 return. But the good news is if they paid you too much they won't take it back.
When you go through the federal review tab it will ask how much you got if any. Or to enter your stimulus payment in TurboTax, type "stimulus" (without the quotes) in the Search box, then click the link that says "Jump to stimulus." If you didn't get one or both enter a 0 and not leave it blank or it won't calculate it.